Divine Repetition

Often expressions are repeated in scriptures to stress their importance or to unfold the meaning step by step.

Tat tvamasi-"thou art that"  is wellk-nown Mahavakya pronounced Nine times in Chhandogya Upanishad (Adhyaya 6 Khanda 9) by Uddalaka Aruni while unfolding Ultimate to his son Shwetaketu

Etad vai tat ‘This is indeed That’ is pronounced Twelve times by Yamadharma in Kathopanishad (Adhyay 2) while enlightening Nachiketa

These repetitions were not for any verbal acrobatics or aesthetics but pure necessity.

There is a line which repeats in Shrimad Bhagavad Gita.

man-mana bhava mad-bhakto
mad-yaji mam namaskuru
mam evaisyasi yuktvaivam
atmanam mat-parayanah (Bg 9-34)

Engage your mind always in thinking of Me, offer obeisances and worship Me. Being completely absorbed in Me, surely you will come to Me.

man-mana bhava mad-bhakto
mad-yaji mam namaskuru
mam evaisyasi satyam te
pratijane priyo 'si me (Bg 18-65)

Always think of Me and become My devotee. Worship Me and offer your homage unto Me. Thus you will come to Me without fail. I promise you this because you are My very dear friend.

It is an irrevocable commitment made by lord.

Bhagavadgita is essence of Upanishadas. It is natural they show similarity. Similarity in the following two verses is striking

Kathopanishad 1.2.18 & 19

na jayate mriyate va vipashchin

    nayam kutashchinna babhuva kashchit.

ajo nityah shashvatoayam purano

    na hanyate hanyamane sharIre .. 18..

The Spirit is not born; It does not die. It has not sprung from anything; nothing has sprung from It. Birthless, eternal, everlasting and ancient, It is not killed when the body is killed.

hanta chenmanyate hantun hatashchenmanyate hatam

ubhau tau na vijanIto nayam hanti na hanyate .. 19..

If the killer thinks he kills and if the killed man thinks he is killed, neither of these apprehends aright. The Self kills not, nor is It killed.

Bhagavad Gita 2.20 & 19

na jayate mriyate va kadachin

        nayam bhutva bhavita va na bhuyah

ajo nityah shashvato.ayam purano

        na hanyate hanyamane sharire .. 2\-20..

The Spirit is neither born nor does it die at any time. It does not come into being, or cease to exist. Birthless, eternal, everlasting and ancient, It is not killed when the body is killed.

ya enam vetti hantaram yashchainam manyate hatam

ubhau tau na vijanito nayam hanti na hanyate .. 2\-19..

The one who thinks that the he is a slayer, and the one who thinks the he is slain, both are ignorant. Because the Self neither slays nor is slain.

Two great poets Valmiki and Veda Vyasa expressed exactly same thing

Sulabhaah purushaa rajan satatam priya vaadinah

Apriyasya cha pathhyasya vakta shrota cha durlabah

(Persons pleasing in speech are easy to find; it is difficult to find one who speaks or listens to the bitter, but wholesome, truth).

Very good verse. What is special about it? 

It appears twice in Valmiki Ramayana- Maricha to Ravana in Aranya Kanda (Sarga 37, verse 2) and Bibhishana to Ravana in Yuddha Kanda (Sarga 16, Verse 21) and also in Mahabharata Vidura to Dhritarashtra in Udyoga Parva (Adhyaya 37, Verse 15)!!